This invention relates to an elastic paint composition. More particularly, the present novel composition is applied to substrates such as the exterior surfaces of buildings, wherein the outer surface of the paint coating hardens by photo-reaction with ultraviolet rays or by oxidation with air, while the under-surface of the paint coating retains enough elasticity (flowability) to cover any cracks which are present or may develop on the substrate, which contributes to maintaining a good appearance.
The elastic paint composition of this invention may include conventional additives mixed in to the paint compositions, such as inorganic fillers, thickeners, dispersing agents, colorants, defoamants, preservatives, fungicides, and the like.
The exterior surfaces such as roofs and walls of buildings are mainly coated with water-proofing paint in order to prevent water from leaking through cracks that may occur or widen on the substrate. These waterproofing paints are composed mainly of elastomers, such as chloroprene, urethane, or acrylic rubbers, which are used to cover newly developed cracks on concrete substrates. Acrylic rubbers are preferred due to their superior weather-resistance properties. Preferably, the elastomers having a low glass transition temperature (Tg) are used because their elasticity is hardly affected over a wide range of ambient temperatures. Unfortunately, these paints tend to be sticky (tacky), and dust particles in the air tend to cling to their surface, thereby eventually spoiling their initial good appearance.
In order to keep a good appearance on the coated surfaces for a long period of time, the paints have to (a) be soil-resistance, (b) retain luster on their surface, and (c) avoid discoloration as much as possible. There have been various attempts to incorporate these desirable properties into paints, but each of them has its own disadvantages. The prior art discloses a method of coating an elastomer-based paint with a hardenable resin-based paint in order to prevent the soiling of the surface. This method comprises applying an elastomeric paint, having a Tg of -20.degree. C., on an area previously coated with a primer. The paint is applied at a rate of about 1 kg/m.sup.2 and allowed to dry for up to one day. After the paint has dried, a hardenable resin is applied having a Tg of at least 0.degree. C. Alternatively, a rubber-based (less sticky) resin may be applied on the elastomeric paint at a rate of 0.2 kg/m.sup.2. However, the overcasting cannot follow the elasticity of the undercoating. As a result there frequently appears cracks in the overcoating which will also cause the undercoating to crack. This method of double coating is both time-and labor consuming.
Japanese patent application laid-open publication No. Sho 59-193968 discloses a method for preventing applied paint from soiling, by applying a mixture of an acrylic type oligomer with an acryloyl or methacryloyl radical and a soft acrylate-based resin dissolved in an organic solvent as a coating which is used for 10 days under standard conditions. The problem with this method is that the surface of the mixture does not harden as well as expected. The resin is so uniformly mixed with the oligomer that it is difficult to harden only the surface. This kind of mixed paint is inferior in luster-retainability and becomes soiled comparatively easily.
The prior art also discloses a method for preventing an applied paint from soiling by applying a soft resin-based paint mixed with an oxidation-hardenable oligomer and allowing the mixture to harden in the air. This method has a similar problem to the method disclosed in Japanese Patent application Publication No. 59-193968, because the paint is too uniformly-mixed with the oligomer to form a hard crust thereon.
Accordingly, it is an object of this invention to provide a paint composition coating that retains its luster and soil-resistance on its hard upper surface, and also retains the flowability or flexibility of the under-surface of the paint coating which covers cracks which may develop or are present on the substrate.